Plastic parisons or preforms are conventionally reheated and formed into hollow plastic articles by stretch blow molding in a blow mold using a stretch rod and high pressure fluid.
The preforms are preheated in ovens immediately prior to being transferred into a blow mold. Incorporated with the blow mold are stretch rods which typically enter the preform from its open end and extend during the blow stage to cause biaxial orientation of the blown article. The motion of the stretch rod should be synchronized with the blowing of the high pressure fluid so that the preform is stretched along its longitudinal axis by the rod and simultaneously or sequentially stretched perpendicular to this axis by the pressure of the incoming fluid to orient its molecules for greater strength.
Stretch rods have been typically moved mechanically with cams driven by a motor or by direct actuation of hydraulic cylinders. Both these methods while offering some degree of control of the speed of the stretch rod motion and the rate of change of this speed during the stretch rod stroke are not easily or accurately able to synchronize this motion with the blowing action.
Stretch blow molding machines are shown in U.S. Pat. Nos. 4,522,581, 4,690,633, 4,128,383 and 4,818,212. The apparatus described in U.S. Pat. No. 4,690,633 carries preforms on pallets from a loading station to a finishing or blowing station via a conditioning station. The preform is blown at the blowing station and then the blown article moved to an unloading station.
Accordingly, it is a principal object of the present invention to provide an apparatus for forming a stretch blow molded hollow plastic article.
It is a further object of the present invention to provide an apparatus as aforesaid which synchronizes the motion of the stretch rod with the blowing of the high pressure fluid into the preform, and which can do this in an efficient and reliable manner.
Further objects and advantages of the present invention will appear hereinbelow.